Microporous membranes can be used as battery separator film (“BSF”) in, e.g., primary and secondary lithium batteries, lithium polymer batteries, nickel-hydrogen batteries, nickel-cadmium batteries, nickel-zinc batteries, silver-zinc secondary batteries, etc. When microporous polyolefin membranes are used for battery separators, particularly lithium ion battery separators, the membranes' characteristics significantly affect the battery's properties, productivity and performance. Accordingly, it is desirable for the microporous membrane to have a relatively high meltdown temperature and be electrochemically-stable during battery manufacturing and use, particularly in batteries subjected to relatively high temperatures as can occur under overcharge or rapid-discharge conditions.
Electrochemically-stable microporous membranes having a relatively high meltdown temperature have been produced from polypropylene. For example, Japanese Patent Application JP10-279718 discloses multi-layer microporous membranes having outer layers comprising polypropylene, the polypropylene content of the outer layers being in the range of 80 wt. % to 100 wt. %. While these membranes have a relatively high meltdown temperature and electrochemical stability, the large amount of polypropylene in the outer layer undesirably lessens the membrane's electrolyte affinity and increases the membrane's moisture retention.
Japanese Patent Application JP2009-45774 discloses a multi-layer microporous film comprising first layer including polypropylene polymer and second layer, wherein first layer includes thermoplastic polymer having crystallization melt peak temperature ≧170.0° C., e.g., polyester polymer or polymethylpentene (PMP) polymer. As a result, the film improves breakdown property compared to conventional microporous film, but its electrochemical stability does not become enough state because the skin layers include polypropylene polymer.
Japanese Patent Application JP2003-142064 and Japanese Patent Application JP2004-224915 disclose microporous membrane comprising 4-metyl-1-penten copolymer or polymetylpentene polymer. There membrane improve heat-resistant, but the balance of it and other properties is worse.
Japanese Patent Application JP8-250097 discloses that a multi layer membrane comprises three layers (A/B/A), wherein layer A contains poly(4-methylpenten-1). This multi layer membrane has an improved shutdown property, but the balance of it and other properties is worse.
There is therefore a need for microporous membranes having an improved balance of electrochemical stability, high meltdown temperature, high electrolyte affinity, and low moisture retention.